The important biological roles that peptides and polypeptides play as hormones, enzyme inhibitors, substrates, neurotransmitters, and neuromediators has led to the widespread use of peptides and peptide mimetics in medicinal chemistry as therapeutic agents. The peptide's bioactive conformation, combining structural elements such as alpha-helices, beta-sheets, turns, and/or loops, is important as it allows for selective biological recognition of receptors or enzymes, thereby influencing cell-cell communication and/or controlling vital cell functions, such as metabolism, immune defense, and reproduction (Babine et al., Chem. Rev. (1997) 97:1359). The alpha-helix is one of the major structural components of peptides. However, alpha-helical peptides have a propensity for unraveling and forming random coils, which are, in most cases, biologically less active, or even inactive, and are highly susceptible to proteolytic degradation.
Many research groups have developed strategies for the design and synthesis of more robust peptides as therapeutics. For example, one strategy has been to incorporate more robust functionalities into the peptide chain while still maintaining the peptide's unique conformation and secondary structure (see, for example, Gante et al., Angew. Chem. Int. Ed. Engl. (1994) 33:1699-1720; Liskamp et al., Recl. Trav. Chim. Pays-Bas (1994) 113:1; Giannis et al., Angew. Chem. Int. Ed. Engl. (1993) 32:1244; P. D. Bailey, Peptide Chemistry, Wiley, New York, 1990, p. 182; and references cited therein). Another approach has been to stabilize the peptide via covalent crosslinks (see, for example, Phelan et al., J. Am. Chem. Soc. (1997) 119:455; Leuc et al., Proc. Nat'l. Acad. Sci. USA (2003) 100:11273; Bracken et al., J. Am. Chem. Soc. (1994) 116:6432; and Yan et al., Bioorg. Med. Chem. (2004) 14:1403). Crosslinking a polypeptide predisposed to have an alpha-helical secondary structure can constrain the polypeptide to its native alpha-helical conformation. The constrained secondary structure may, for example, increase the peptide's resistance to proteolytic cleavage, may increase the peptide's hydrophobicity, may allow for better penetration of the peptide into the target cell (e.g., through an energy-dependent transport mechanism such as pinocytosis), and/or may lead to an improvement in the peptide's biological activity relative to the corresponding uncrosslinked peptide. Therefore, there remains a need and interest in developing new crosslinked alpha-helical polypeptides as therapeutic agents as well as research tools.